Inductive wireless power transfer (IWPT) enables short range wireless power transfer from a power source to a load through inductive coupling. One application of inductive wireless power transfer is in the powering and charging portable consumer electronic devices, such as cell phones, smart phones, tablets, and laptop computers. In such an application, a portable device including an inductive coil is placed on a base station that also includes an inductive coil. The power source drives the inductive coil in the base station causing a transfer of electromagnetic energy from the power source inductive coil to the portable device inductive coil. The transferred energy is then used to power the portable device, e.g., to charge the batteries of portable device. Two IWPT techniques that are employed today in commercial products include tightly coupled inductive charging and loosely coupled charging.
A tightly coupled charging system works similar to a transformer and relies on a strong magnetic linkage, i.e., mutual inductance, between the source and load coils. To achieve the strong magnetic linkage, the load inductive coil may be placed in close proximity and in alignment with the power source inductive coil. Commercial examples of tightly couple charging systems include the Qi standard developed by the Wireless Power Consortium, and the Powermat™ standard adopted by the Power Matters Alliance (PMA).
In a loosely coupled charging system, efficient energy transfer is achieved through magnetic resonance of the load and source inductive coils rather than through strong magnetic linkage. Because loosely coupled charging systems do not rely on strong magnetic linkage between the coils, proximity and alignment of the coils is not as critical. A commercial example of a loosely coupled (or resonant) charging system is put forth in the Alliance for Wireless Power (A4WP) standard.
The different techniques (e.g., tight or loose coupling) may benefit from different design parameters to work efficiently. Such parameters that differ between the different techniques may include coil size, operating frequency, distance between coils, coil alignment, ferrite materials, shielding materials, etc. As such, a mobile device or appliance designed for one IWPT system may not work with a power source designed for a different IWPT system.